Three-phase voltage source inverters (VSI's) are generally used to convert DC power into three-phase AC power. Typically, the three-phase output voltages are sinusoidal waveforms spaced 120 degrees apart, to be compatible with a wide variety of applications requiring conventional AC power. In general, the output power frequencies commonly used are 50, 60, and 400 hertz, but other frequencies could be used as well. One current example of an inverter application is the electric or hybrid automobile, where a DC power source, such as a battery, fuel cell array, or other equivalent device, is converted into an AC power supply for various internal control functions, including the propulsion system.
The quality of an inverter is generally determined by its output voltage and frequency stability, and by the total harmonic distortion of its output waveforms. In addition, a high quality inverter should maintain its output stability in the presence of load current variations and load imbalances.
In the case of unbalanced loads, the 4-leg three-phase inverter topology is generally considered to offer superior performance than a 3-leg three-phase topology. That is, with an unbalanced load, the 3-phase output currents from an inverter will generally not add up to zero, as they would in a 3-leg balanced load situation. Therefore, a fourth (neutral) leg is typically added to accommodate the imbalance in current flow caused by an unbalanced load. If a neutral is not used with an unbalanced load, voltage imbalances may occur at the load terminals, and the output power quality may be adversely affected.
The operational functions of a typical inverter are generally controlled by drive signals from an automatic controller. The controller and inverter are usually implemented as a closed loop control system, with the inverter output being sampled to provide regulating feedback signals to the controller. The feedback signals typically include samples of the output voltage and current signals, and can also include harmonics of the fundamental output frequency.
The output frequency harmonics are usually suppressed by a 3-phase inductor-capacitor (L-C) filter, which is normally connected at the output of the inverter. However, a typical L-C filter has very low component resistance, and may exhibit under-damped behavior. This behavior can lead to filter oscillations as a result of sudden changes in the inverter load, and can create distortion or over-voltages on the load. Moreover, the typical voltage control loop response of an inverter controller may be inadequate to compensate for this type of L-C filter oscillation.
One method of mitigating the oscillation tendency of an under-damped L-C filter is to add damping resistors in the filter circuit. However, resistive damping will generally have a degrading effect on inverter efficiency, and can also complicate the thermal management of the inverter.
Accordingly, it is desirable to provide an inverter controller with a damping control scheme that will reduce the tendency of the L-C output filter to oscillate without degrading the efficiency of the inverter. In addition, it is desirable to provide an inverter controller with a damping scheme that will also improve the transient performance of the inverter. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.